The present invention relates to an apparatus and method for analyzing particulate matter (hereinafter referred to as PM) contained in a gas discharged, for example, from diesel engine. The present. invention further relates to a carbon differentiating and analyzing apparatus and method capable of differentiating and analyzing organic carbon and elemental carbon contained in a sample.
As a procedure for determining PM contained in an emission of diesel engine, there is known in general a filter weighing method for quantitative analysis based on the weight difference from the filter before capturing PM which comprises diluting a high temperature emission discharged from the diesel engine with clean air, capturing PM with a filter by inhaling the diluted emission by a fixed capacity, and weighing the filter with a precision balance or the like.
However, in the above filter weighing method, the large effect of water content absorbed by the filter leads to measurement error. Constant temperature and constant moisture processing is required to maintain the water content at a constant in the filter before and after capture. Furthermore, in determining the emission of low concentration PM, it is necessary to weigh accurately PM of 0.1 mg captured on a filter of for example 200 mg. Thus, there is a problem such that the measurement error of the weight of the filter itself gives a significant effect on the measurement error of the PM weight.
Against the above, as shown in U.S. Pat. Nos. 5,110,747, 5,196,170, 5,279,970, and 5,401,468 publications, there is a procedure of heating a filter which has captured PM in a heating furnace by elevating the temperature stepwise, oxidizing PM, and determining PM with a gas analyzer.
However, the majority of PM is constituted by the inorganic carbon called dry soot (hereinafter, to be dry soot), hydrocarbon called SOF (soluble organic fraction) (hereinafter, to be SOF), and sulfuric acid hydrate called sulfate (hereinafter, to be sulfate), and depending on the procedure described in the above publication, separation of high boiling SOF and reduction of sulfate in an oxidation atmosphere are difficult. Accordingly, it has been difficult to measure individually the concentration or weight of the dry soot, SOF and sulfate by separating them which occupy the majority part in PM.
The present invention has been made in consideration of the matters described above, and its object is to provide a method for analyzing simply and with high precision the particulate matter in an engine emission and an apparatus therefor by which dry soot, SOF and sulfate in PM contained in an engine emission can be fractionated individually even when in a minute amount.
Further, elemental carbon, which is one of the carbon components contained in TSP (total suspended particulate matter) in atmospheric air, is involved in the oxidation of SO2 into SO42xe2x88x92, and may cause climatic change depending on its concentration. One technique for analyzing the elemental carbon is to thermally separate between elemental carbon and organic carbon, which is the other carbon component contained in TSP.
FIG. 11 is a schematic view showing a construction of a conventional carbon differentiating and analyzing apparatus. Referring to FIG. 11, a thermal decomposition tube 61 includes, on one end, a sample entrance 62 having a lid freely openable and closable for inserting a sample S, and on the other end, a gas exit 63. The thermal decomposition tube 61 further includes, on the sample entrance 62 side thereof (hereafter to be referred to as upstream side), an inlet 64 of carrier gas CG. A heater 65 is wound around the outer circumference of the thermal decomposition tube 61 on the downstream side of the carrier gas inlet 64, and the temperature of the heater 65 can be set to both a low temperature and a high temperature. A CO2 analyzer 66 is connected to the gas exit 63 via a suitable passageway (not illustrated). Further, the sample S is mounted on a sample boat 67.
Operation of the carbon differentiating and analyzing apparatus having the aforesaid construction will be described. First, a sample boat 67 having a sample S mounted thereon is set at a predetermined position in the thermal decomposition tube 61. In this state, the heater 65 is set at a low temperature (for example, about 400xc2x0 C.) to heat the sample S, whereby the organic carbon contained in the sample S is evaporated to become CO2 and then carried to the CO2 analyzer 66 by the carrier gas CG to be measured for CO2 concentration. Next, the heater 65 is set at a high temperature (for example, about 1000xc2x0 C.) to further heat the sample S, whereby the elemental carbon contained in the sample S is thermally decomposed to become CO2 and then carried to the CO2 analyzer 66 by the carrier gas CG to be measured for CO2 concentration. Based on the CO2 concentration obtained in each of the aforesaid measurements, the amount of the organic carbon and the amount of the elemental carbon in the sample S can be determined.
However, in the case of the aforesaid conventional carbon differentiating and analyzing apparatus, only a single heater 65 is used. Thus, it takes time to set the heater 65 from a low temperature to a high temperature, and further time is needed to set the heater 65 from a high temperature to a low temperature. Moreover, since it is difficult to detect the HC component evaporating at a low temperature with the use of the CO2 analyzer, a HC gas sensor is further needed. Also, CO is generated to cause errors if the oxidation does not sufficiently takes place because of insufficient temperature.
The present invention has been made in order to take the above-mentioned matters into account. An object thereof is to provide a carbon differentiating and analyzing apparatus capable of differentiating and analyzing, with precision, the organic carbon and the elemental carbon contained in a sample, and capable of performing a predetermined analysis with high precision in a short time by reducing the period of time required for analysis.
In order to attain the above object, the method for analyzing PM in gas of the present invention comprises providing a filter which has caught PM contained in engine emission in a heating furnace, firstly heating the filter at a predetermined temperature while passing an inert gas into a heating furnace to evaporate the SOF and sulfate. contained in PM, oxidizing the evaporated SOF into CO2 and reducing the evaporated sulfate into SO2, analyzing the CO2 and SO2 with a gas analyzer unit, and thereafter, heating the filter while passing oxygen into the heating furnace to oxidize the PM remaining on said filter to have it generate CO2, and analyzing the CO2 with the gas analyzer unit.
As a concrete apparatus for executing the above analyzing method, there is used in the present invention an apparatus comprising a gas feeder for selectively feeding an inert gas or oxygen to a heating furnace, a heating furnace for heating at a predetermined temperature a filter which has captured the PM contained in the engine emission in a condition where the inert gas or oxygen is fed, an oxidation reduction processor for oxidizing or reducing the gas generated by heating, and a gas analyzer unit for determining the concentrations of CO and SO under supply of a gas from the oxidation reduction processor.
According to the method for analyzing particulate matter in gas of the present invention, a filter is installed in a flow passage in which an emission from an engine is passed. The emission is passed at a predetermined flow amount and the PM in the emission is captured by a filter. The filter is provided in a heating furnace which is maintained for example at 1000xc2x0 C. Firstly, the filter is heated while an inert gas such as nitrogen gas is passed into the heating furnace to gasify SOF and sulfate in PM, and the evaporated SOF is oxidized into CO2 and the evaporated sulfate is reduced into SO2. Then, the CO2 and SO2 are analyzed with a gas analyzer unit to obtain the CO2 concentration and SO2 concentration. The CO2 concentration and SO2 concentration are proportionate to the amount of the SOF and the sulfate in PM, respectively. From the CO2 concentration and SO2 concentration and the total flow amount of the inert gas, the weight of CO2 and SO2 are obtained. Based on the weight of CO2 and SO2, the weight of SOF and sulfate captured by the filter are obtained.
Subsequently, while flowing oxygen into the heating furnace, the filter is heated to oxidize the PM remaining on the filter (major part is dry soot) to generate CO2, and the CO2 is analyzed by the gas analyzer unit to obtain the CO2 concentration. The CO2 concentration is proportional to the amount of the dry soot in PM, and the weight of CO2 is obtained from the CO2 concentration and the total flow of oxygen. Based on the weight of CO2, the weight of the dry soot captured by the filter is obtained.
In the above method for analyzing PM in the gas, the filter is heated by passing an inert gas into the heating furnace. Firstly, low temperature heating is effected to an extent that the SOF of low boiling temperature is evaporated. Thereafter, high temperature heating is effected to an extent that the SOF of high boiling temperature is evaporated. When such practice is performed, determination can be made by distinguishing between the high boiling point SOF and the low boiling point SOF.
Further, in the aforesaid method of analyzing the PM contained in the gas, the aforesaid sulfate may be passed through heated quartz fibers to reduce the evaporated sulfate into SO2. If this is carried out, the sulfate can be heated efficiently and with certainty so that the sulfate can be reduced to SO2 with more certainty.
Further, in order to achieve the aforesaid object, the present invention provides a carbon differentiating and analyzing apparatus including a thermal decomposition tube having a sample entrance formed at one end and a gas exit formed at the other end. A CO2 analyzer is connected to the gas exit side, wherein the thermal decomposition tube includes, successively from its upstream side, a carrier gas inlet, a low temperature heating portion for evaporating organic carbon in the sample, and a high temperature heating portion for thermally decomposing elemental carbon in the sample, whereby the organic carbon is separated in the low temperature heating portion and the elemental carbon is separated in the high temperature heating portion.
The aforesaid carbon differentiating and analyzing apparatus eliminates the time required for heating and cooling during carbon differentiation and analysis.
A high temperature oxidizing portion is provided midway between the high temperature heating portion and the gas exit for oxidizing generated carbon compounds such as HC and CO into CO2, HC and CO prevented from flowing into the CO2 analyzer as they are.
Also, if a tube for introducing oxygen or air is connected at a site between the high temperature heating portion and the high temperature oxidizing portion of the thermal decomposing tube, low temperature CO in the presence of oxygen is completely converted into CO2, whereby errors of the CO2 analyzer caused by the generation of CO can be eliminated.
Furthermore, if the tube for introducing oxygen or air and a neighborhood of a site of connection between the tube and the thermal decomposition tube are heated, the temperature of gas generated by introduction of oxygen or air is prevented from decreasing, and the CO2 concentration can be measured with a higher precision.
Still further, if a switching valve is provided at a site between the gas exit of the thermal decomposition tube and the CO2 analyzer, gases that should not flow into the CO2 analyzer, such as air flowing in from the outside and gas that is passed for purging the inside of the thermal decomposition tube, can be discharged to the outside.